In principle, CMOS image sensors provide a number of advantages over CCD image sensors. The CMOS yields are significantly better than those of the CCD fabrication process. In addition, the minimum noise levels achievable with CMOS-based sensors are substantially lower than those that can be obtained with CCDs. Finally, the image stored in a CMOS-based image sensor can be readout without destroying the image.
CMOS sensors suffer from temporal noise. Schemes for reducing temporal noise are known to the art. For example, U.S. Pat. No. 6,424,375, which is hereby incorporated by reference, describes an active pixel reset system that reduces the effects of temporal noise. However, the pixel design discussed in this patent requires a CMOS pixel having a number of additional components that are used to reset the voltage on a photodiode prior to accumulating an image. This additional circuitry increases the size of the pixel, and hence, increases the cost of an imaging array utilizing this design.
In addition, there is a tradeoff between the degree of noise reduction and the time needed to reset the pixel. To provide high noise reduction, the bandwidth of the reset circuit must be limited. This increases the time needed for the photodiode to stabilize at the reset potential. If the bandwidth is increased to provide a shorter reset time, the noise level increases. Hence, this design cannot provide both short reset times and high noise reduction.